protected void DayPilotBubble1_RenderEventBubble(object sender, RenderEventBubbleEventArgs e)
{
e.InnerHTML = "<b>Event details</b><br />Here is the right place to show details about the event with ID: " + e.Value + ". This text is loaded dynamically from the server.";
}

ASP.NET MVC

The bubble object must be declared before it is used (Html.DayPilotBubble helper must be above the Html.DayPilotScheduler helper).